Observation of fractional states in the the three-dimensional quantum Hall regime of HfTe₅

Interacting electrons in two dimensions can bind magnetic flux lines to form composite quasiparticles with fractional electric charge. Existence of such states manifests itself by the occurrence of the Fractional Quantum Hall Effect (FQHE). Although similar composite quasiparticles have been predicted to occur in three dimensions, their experimental realization remained elusive. In this talk, we report the observation of fractional plateaus in the Hall conductivity of the bulk semimetal HfTe₅. The plateaus are accompanied by Shubnikov-de-Haas minima of the longitudinal electrical resistivity. The height of the Hall plateaus is given by twice the Fermi wave vector in the direction of the applied magnetic field and scales with integer and particular fractional multiples of the conductance quantum. Our findings are consistent with the 3D FQHE, suggesting the existence of quasiparticles with fractional electric charge in a 3D crystal.